Flatpak is a universal application packaging standard for Linux. It allows devs to create a single application that gets bundled with all necessary dependencies including versioning.
These apps run in their own semi-isolated “container” which makes immutable distros possible. (Distros like Fedora Silverblue that are effectively impossible to break by installing or removing critical system files.)
This means that a Linux app doesn’t have to have a .deb version, an .rpm version, or be pre-compiled for any other distros. A user can simply go to Flathub, (the main repository for Flatpak apps), download the flatpak, and install it on their distro of choice.
It’s quickly becoming the most popular way for users to install apps on Linux because it’s so easy and quick. But there are a few downsides like size on disk, first party verification, per-distro optimizations, and the centralization of application sources. That’s why some users aren’t fully endorsing or embracing how popular they are becoming.